This study introduces a "leaching-carbonation" method for converting magnesium slag (MS) into two valuable byproducts: high-purity spherical vaterite calcium carbonate (VCC) derived from the carbonation of filtrate and leached solid residue (LSR) rich in amorphous silica gel. The investigation delves into the performance and microstructure of cement incorporating MS, VCC, and LSR. Results indicated that MS and LSR decreased the flowability of cement mortar, while VCC significantly enhanced it compared to the control sample. MS negatively affected compressive strength and water absorption, whereas LSR and VCC improved both aspects in comparison to MS. Although MS, LSR and VCC shortened induction period of cement paste, both LSR and VCC improved the cumulative heat release compared to the control sample. Additionally, a new hydration product, monocarbonate, was formed in the VCC sample. The presence of amorphous silica gel in LSR exhibited pozzolanic reactivity and could react with Ca(OH)2 to form C-S-H gel. Furthermore, both LSR and VCC samples exhibited a refined pore size distribution compared to the control and MS samples. In summary, the obtained results offer a promising technology for maximization of the valuable utilization of MS.